British patent application No. 2082028 describes a clamping device for arc furnace electrodes which includes three clamping jaws (2) for radially surrounding an electrode, each jaw spanning a 120.degree. arc, and having axial grooves (10) in which graphite bars (8) are arranged. By pressing the clamping jaws (2) the graphite bars (8) are pressed against the body of the electrode (6), establishing mechanical and electrical contact therebetween.
Referring to FIG. 2 of British application No. 2082028, the graphite bars (8) have a length which is many times their diameter. Often under usual contact pressures, the risk of breakage for these long, thin graphite bars is relatively high.
As long as the electrode (6) as shown in British application No. 2082028 is in a clamped position, it does not matter whether the graphite bars are still intact or whether they have broken. However, if clamping is released and the electrode subsequently mounted again, problems may arise as broken parts may wedge. As a consequence, the contact between the graphite bars and the electrode will considerably deteriorate.
A further drawback of clamping device, typical of those shown in British application No. 2082028 is that each individual graphite bar has to be affixed to one of the clamping jaws in a relatively complicated manner illustrated in FIGS. 2 and 3. Furthermore, the great number of contact areas between clamping jaws and graphite bars on one hand and between the latter and the electrode body on the other can have an unfavourable effect upon current conduction.
Due to the fact that the graphite bars may break it is practically impossible to change the place of clamping between clamping jaws and electrode, for if the clamping jaws are released there is the danger that any broken parts will become wedged.
European patent application No. 75534, corresponding to U.S. Pat. No. 4,446,561, describes an electrode having graphite segments along its periphery at two axially moved clamping positions which are firmly connected to the metal shaft of the electrode.
One purpose of the graphite segments is to prevent direct contact between the metallic clamping jaws and the upper shaft of the electrode, as far as such jaws and shaft consist of metal, since such contact would lead to weld connections during operation and, consequently, to a rigid connection between the electrode and the support arm.
While the proposal put forth in European publication No. 75534 is satisfactory from the technical viewpoint, it is, however, necessary that the graphite segments be in clamping positions possibly not required during operation so that improvements are possible with regard to costs.